orangeblossom

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Jan 23, 2011
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Equation for Vd = amt of drug in body / plasma drug concentration

But in my notes (from usmle rx or DIT... I can't remember), I wrote "if P.O. administration of drug given" and my calculations show Vd = 250 * F = 250 * 0.5 = 125 L.

(250 was previously calculated.)

So my question is, do you ever incorporate the bioavailability to determine Vd? Or am I misinterpreting my notes?
 
Aug 9, 2014
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Yes. If it's not 100% bioavailable, then x% isn't really "in the body".
 
Jun 30, 2015
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Yes. If it's not 100% bioavailable, then x% isn't really "in the body".
Interesting point.. never have thought about this in the way you mentioned it. Could you give an example?

Bioavailability is a fraction f of a dose that reaches the systemic circulation.
f = AUC PO (f.e.) / AUC IV (max possible)
But if you get a f of 0.4, this doesn't necessarily mean, that 0.6 of the drug is redistributed somewhere else. The 0.6 can be because of the absorption phase...

If you would just look at the elimination curve of the drug, which was given IV or PO alone and then extrapolate, you can calculate C0, from which you can then calculate Vd. (Vd = D/C0)
 
Aug 9, 2014
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Interesting point.. never have thought about this in the way you mentioned it. Could you give an example?

Bioavailability is a fraction f of a dose that reaches the systemic circulation.

But if you get a f of 0.4, this doesn't necessarily mean, that 0.6 of the drug is redistributed somewhere else. The 0.6 can be because of the absorption phase...

If you would just look at the elimination curve of the drug, which was given IV or PO alone and then extrapolate, you can calculate C0, from which you can then calculate Vd. (Vd = D/C0)
I might have an incorrect understanding of bioavailability, but I thought it was the % dose that entered circulation. What's more, my understanding of Vd is that it's basically the relationship between what stays in the blood vs what distributes from the blood and into tissues. So technically for a drug with 50% bioavailability, since only 50% gets absorbed into circulation, only 50% of the total dose can be used in the Vd calculation since only 50% actually has the opportunity to be in the blood or be in the tissues.
 

Jabbed

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Jan 30, 2013
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Interesting point.. never have thought about this in the way you mentioned it. Could you give an example?

Bioavailability is a fraction f of a dose that reaches the systemic circulation.

But if you get a f of 0.4, this doesn't necessarily mean, that 0.6 of the drug is redistributed somewhere else. The 0.6 can be because of the absorption phase...

If you would just look at the elimination curve of the drug, which was given IV or PO alone and then extrapolate, you can calculate C0, from which you can then calculate Vd. (Vd = D/C0)
He made a simplification that's accurate enough for our purposes. The 'area under the curve' ratio is just a slightly more sophisticated way of saying "what percentage of this drug will actually be able to exert a pharmacologic effect?"